The present invention relates to a method of making a moulded plastics article, especially where the plastics is an epoxy resin cured with an anhydride hardener.
Epoxy resins can be used to make moulded articles by what is called the Automatic Pressure Gelation Process (APG). In this process which is described for example in GB 1323343 and EP 0333456 an epoxy resin and a curing agent are mixed at a temperature at which they are liquid, usually 40.degree.-60.degree. C. The mixture is then passed, under slight pressure, into a mould which is at a high enough temperature for gelling and curing to take place. Further mixture is supplied to the mould under the application of pressure to compensate for shrinkage of the composition until the composition has set.
Depending on whether a thick-walled or thin-walled casting is being made the technique is slightly different. In order to produce a thick-walled cured plastics moulding a pre-heated liquid casting resin composition which is capable of setting within a period of three to sixty minutes is poured into a preheated mould substantially without the application of pressure so as substantially to fill the mould, the temperature of the mould being sufficient to initiate curing of the resin composition and the temperature of the composition being at least 10% below the temperature of the mould, said temperatures being measured in degrees centigrade, the temperature of the mould and the temperature of the resin composition being selected such that the temperature in the centre of the moulding composition will not reach the temperature of the composition at the mould wall until the composition has set sufficiently to enable it to be removed from the mould, and further composition is supplied to the mould with the application of pressure for compensating for shrinkage of the composition until the composition has set, whereafter the set moulding is removed from the mould.
In order to produce a thin-walled cured plastics moulding a pre-heated and de-gassed resin composition capable of setting within sixty minutes is supplied under pressure to a mould cavity of the shape of the moulding in a hotter pre-heated mould so as substantially to fill the mould cavity, gases are exhausted from the mould cavity as it is filled and are restrained from entering into the mould cavity during setting of the composition, and further composition is supplied under pressure to the mould cavity until the composition has set so as to compensate for shrinking of the composition, wherein the composition is pre-heated to a temperature of 50.degree.-120.degree. C. and is supplied to the mould cavity under a pressure of at least 4 psi (280 g/cm.sup.2), the maximum temperature of the mould cavity is 120.degree.-170.degree. C. and the temperature of the mould is controlled to maintain a substantially linearly increasing temperature profile in the composition away from the entry for the composition into the mould cavity to peripheral regions of the cavity remote from the entry, whereby setting of the composition progresses through the mould cavity from locations remote from the entry back to the entry.
The typical epoxy resin/anhydride casting resin system comprises about 25-45% by volume of epoxy resin, 12-30% by volume of anhydride hardener and 30-65% by volume of mineral filler, together with minor levels of cure accelerator and other additives. As supplied to the processor the composition may be preformulated to reduce the number of parts needing to be mixed prior to use. In practice 2-part compositions are most popular although 3- or even 4-part compositions are not uncommon.
There are numerous disadvantages to this general form of presentation of the resin system.
Compounding of multi component blends is costly of time and labour. It introduces a high risk of operator error, particularly since large numbers of small batches are likely to be required and also because some of the components are used in relatively small amounts. Furthermore, costly facilities are necessary to prevent health hazards from inhalation of mineral fillers.
If compounded as a two part blend, the system is most usually processed through an automatic metering and mixing machine. Such equipment reduces labour and the risk of operator error, but incurs substantial extra capital costs.
Compounding as a two-part system creates difficulties in achieving the high loading of mineral fillers desirable from both cost and technical considerations. The liquid resin component can accommodate only about half as much filler as can the overall resin-hardener mixture. On the other hand, filling the hardener component creates severe problems of settlement during storage because of the very low viscosity of the normally used liquid anhydrides. Furthermore, the separate filling of both resin and hardener is inefficient because it involves an additional processing step with concomitant costs.
When processing the resin to make a moulded article, the resin is first preheated in an oven at about 90.degree. C. The hardener is then added and the mixture is then mixed under vacuum to de-aerate it. The mixing temperature may be about 65.degree. C. and the resulting mixture has a usuable life of about 3 hours at 65.degree. C.
The mixture is then fed into a suitable mould and cured in the mould at 140.degree.-190.degree. C.
We have now found that it is possible to use a stable one-part system in the APG process which avoids the disadvantages of the conventional two or multi-part system and produces moulded articles at least as good.